1. Field of the Invention
The present invention relates to an optical element including an organic light emitting layer and an organic EL (electroluminescence) display having organic electroluminescent elements.
2. Description of the Related Art
According to an organic EL display having a number of pixels each constituted by an organic electroluminescent element (hereinafter, referred to as organic EL element), voltage is applied to each of the organic EL elements, electrons are injected from a cathode thereof and holes are injected from an anode thereof respectively to an organic light emitting layer and light is emitted by causing recombination of electrons and holes in the organic light emitting layer.
As such an organic EL element provided to an organic EL display, for example, there is a single hetero-structure type organic EL element shown by FIG. 9. According to the organic EL element, an anode 2 comprising a transparent conductive film made of ITO (Indium tin oxide) or the like is provided on a transparent substrate 1 of a glass substrate or the like, on which an organic layer 5 comprising a hole transport layer 3 and a light emitting layer 4 and a cathode 6 made of aluminum or the like are provided in this order.
Further, according to the EL element constituted in this way, when positive voltage is applied to the anode 2 and negative voltage is applied to the cathode 6, holes injected from the anode 2 reach the light emitting layer 4 via the hole transport layer 3 and electrons injected from the cathode 6 reach the light emitting layer 4, respectively and recombination of electrons and holes is caused in the light emitting layer 4. At this occasion, light having a predetermined wavelength is generated and is emitted from a side of the transparent substrate 1 to outside as shown by arrow marks in FIG. 9.
Accordingly, by arranging a number of the organic EL elements in, for example, a matrix-like shape, an organic EL display is formed as mentioned above.
FIG. 10 shows an example of such a conventional organic EL display. The organic EL display shown by FIG. 10 is constituted such that a plurality of transparent electrodes 8 in a stripe-like shape (band-like shape) are provided on a transparent substrate 7, an organic layer 9 in a sheet-like shape constituted by laminating a hole transport layer and a light emitting layer are provided on the transparent electrodes 8 and a plurality of cathodes 10 in a stripe-like shape (band-like shape) are provided on the organic layer 9 to be orthogonal to the transparent electrodes 8 and organic EL elements are formed at positions where the transparent electrodes 8 and the cathodes 10 intersect with each other.
FIG. 11 is a view showing other example of a conventional organic EL display. The organic EL display shown by FIG. 11 is constituted such that the transparent electrodes 8 in a stripe-like shape are provided as anodes on the transparent substrate 7, organic layers 11a, 11b and 11c in a stripe-like shape each comprising a hole transport layer and a light emitting layer are provided on the transparent electrodes 8 in a state in which the organic layers and the transparent electrodes 8 are orthogonal to each other and cathodes 12 in a stripe-like shape having dimensions substantially equal to dimensions of the organic layers 11a, (11b and 11c) are provided on the organic layers, 11a, 11d and 11c. In this case, each of the organic layers 11a, 11b and 11c is provided with a light emitting characteristic in correspondence with one of red (R), green (G) and blue (B) by which the organic EL display constitutes a display of full color or multiple color.
An explanation will be given of image display by the color organic EL display shown by FIG. 11. According to the color organic EL display, as shown by FIG. 12, a scanning circuit 13 is connected to the transparent electrodes 8 and a brightness signal circuit 14 is connected to the cathodes 12. Further, the organic layers 11a, 11b and 11c respectively emit light by time-sequentially applying signal voltages to the organic layers 11a, 11b and 11c at positions intersecting with the transparent electrodes 8 and the cathodes 12 by the scanning circuit 13 and the brightness signal circuit 14. Accordingly, the organic EL display functions as an image reproducing apparatus by such a control.
However, there is the following inconvenience in the organic EL display.
In the case in which the organic EL display is driven by, for example, the simple matrix system, when a number of scanning lines is several hundreds, in order to ensure sufficient brightness, current of about 1 A/cm.sup.2 need to flow. Then, in this case, although it differs depending also on the size of the display, current of about 0.5 through 1 A is flowed instantaneously in the transparent electrodes 8 connected to the scanning circuit 13.
Further, the resistance value of ITO which is normally used for the transparent electrode 8 is about 100 times as much as that of a metal of aluminum or the like and its alloy. Accordingly, when large current of about 0.5 through 1 A is flowed as mentioned above, voltage drop in the transparent electrode 8 is increased. Further, when such a large voltage drop is caused in the transparent electrode 8, voltage applied on the respective organic EL elements in the organic EL display becomes nonuniform and the display function of the organic EL display is significantly deteriorated.
That is, when the organic EL display is driven by the simple matrix system, although depending also on the display size, current flowing in the electrodes on the scanning side becomes 100 through 1000 times as much as current flowing in the electrodes on the brightness signal side in view of the drive principle. However, in the case of the organic EL display, large current is flowed in the transparent electrode 8 having high resistance. Therefore, large voltage drop is caused in a transparent conductive film constituting the transparent electrode 8. Voltage applied on the organic layers 11a, 11b and 11c constituting the respective pixels becomes nonuniform by which the display function is deteriorated and power consumption in the transparent electrode 8 is increased.
Further, in the case of the color organic EL display shown by FIG. 11, the organic layers 11a, 11b and 11c are formed over an entire lower face thereof along the length direction of the cathodes 12 formed in a stripe-like shape. Owing to such a structure, brightness signals necessary for respective colors of R, G and B must be provided from the cathodes 12. Therefore, the brightness signal circuit 14 must be connected to the cathodes 12 and the scanning circuit 13 must be connected to the transparent electrodes 8.
Further, as mentioned above, power consumed in the transparent electrodes 8 is increased and accordingly, low power consumption formation in the entire organic EL display is deteriorated. Accordingly, to provide the organic EL display having low power consumption, the resistance of electrodes on the scanning side needs to reduce to thereby reduce voltage drop.
As a measure for reducing the resistance of the scanning side electrodes, there is disclosed a technology in which metal wirings are installed along with the transparent electrodes in JP-A-5-307997. According to the technology, a metal having low resistance is provided at a portion between the transparent electrode and the organic layer to thereby achieve low resistance formation of the scanning electrode.
However, in order to achieve sufficient low resistance formation by such a technology, an area of the metal wiring provided along with the transparent electrode needs to magnify as large as possible. When the area of the metal wiring is magnified in such a manner, the metal wiring covers the organic layers 11a, 11b and 11c constituting light emitting portions, as a result, the light emitting area of the organic EL element is reduced and the light emitting efficiency is deteriorated.
Further, although it is conceivable to achieve low resistance formation by enlarging the film thickness of metal, in such a case, there causes a concern of bringing about shortcircuit between the anode and the cathode and nonuniformity in the film thickness of the organic layer.
Further, according to the conventional organic EL display shown by FIG. 10 and FIG. 11, the organic layer 9 (11) is formed over an entire face or formed continuously along the length direction of the cathode 12. The organic layer 9 (11) is provided with light guiding performance and accordingly, a portion of light generated in the organic layer 9 (11) is propagated in the organic layer 9 (11) and is guided in the transverse direction relative to the transparent substrate 7 as shown by arrow marks C in FIG. 13. Then, the portion of light is attenuated in the guiding operation and the remaining portion is lost by being emitted from peripheral pixels.
That is, it is preferable that all of light generated in the organic layer 9 (11) is emitted to outside of the organic EL display by transmitting through the transparent electrodes 8 and the transparent substrate 7 and utilized as display light. However, according to the structure of the conventional organic EL display, a portion of light generated in the organic layer 9 (11) is not utilized as display light by which the efficiency of utilizing light is deteriorated and the brightness is reduced.
Further, a portion of light transmitted through the organic layer 9 (11) and guided in the transverse direction relative to the transparent substrate 7, is emitted to outside of the transparent substrate 7 by being emitted from the organic layer 9 (11) constituting peripheral pixels by which light originally generated at the peripheral pixels is interfered by the guided light and there also is a concern in which cross talk is caused and color reproducing performance is deteriorated.
Therefore, according to the conventional organic EL display, it is difficult to provide sufficient brightness and the color reproducing performance and in order to provide excellent display function, it is necessary to prevent light transmitting through the organic layer from being guided.
Further, according to the conventional organic EL display, the organic layer 9 (11) at a surrounding of an end portion 8a in the width direction of the transparent electrode 8 is electrically fragile and accordingly, as shown by an arrow mark D in FIG. 13, shortcircuit is caused between the anode (transparent electrode) and the cathode 10 (12) and pixels cannot be selected.